I couldn't find any guideline for this in the spec.
There's a brief mention of this on the page about launch screens:
Placeholder elements, with subtle animation, should be displayed in locations where content will load. Placeholder elements should remain positioned where content will load to avoid unexpected shifts in positioning.
While it may seem like it only applies to screens shown at app launch, the guideline explicitly states: "This launch screen is appropriate for both app launches and activity transitions within an app."
For initial loading, I would also apply using launch screens.
Launch screens can be displayed upon an app’s launch from the home screen when an app loads, instead of displaying a blank screen. Displaying a launch screen can decrease the sense of a long load time, and has the potential to add delight to the user experience.
However, according to material.io, in running applications, launch screens should not be used:
Launch screens shouldn’t be displayed if an app is running.
For running applications, I think it is recommended to use progress indicators:
Progress indicators express an unspecified wait time or display the length of a process.
Progress indicators inform users about the status of ongoing processes, such as loading an app, submitting a form, or saving updates. They communicate an app’s state and indicate available actions, such as whether users can navigate away from the current screen.